Health Insurance for Extensive Travel Inside and Outside US?

I am investigating for my son, 27 years old as of January, who travels extensively for his job. He is currently out of the country without Internet access.
He is about to take a new job that does not offer health coverage / benefits.

Here are his questions:
1- He is only in Texas for a few weeks during the year.
– Do the different categories provide different levels of coverage for a Texas resident that is in-state for only limited periods of time?

2- He has considerable travel outside of the US for his job – possibly multiple months in multiple countries and multiple cities in those countries.
– What type of coverage is available to him: health – general, including emergency room and physical rehab, vision and dental.

3- Once he leaves his current job, how long does he have to enroll in a plan to ensure he does not have a penalty?
– Does the timeframe change if he pays for COBRA?

4- Since I have legal power of attorney, can I enroll him?